O-Metallated yellow iridium(I) complexes, [[graphic omitted])}{PBut2(C6H4OX–2)}](I; X = H or Me), are described. 1H–, 31P–, And 1H–{31P}-INDOR studies show a complex behaviour with temperature, probably due in part to the complexity of the spin systems and to the presence of rotational isomers. A benzene solution of (I; X = H) in air gives, after an induction period, the blood-red paramagnetic iridium(II) complex trans-[[graphic omitted])}2]. The latter reacts with hydrogen to give the dark purple iridium(III) hydride [[graphic omitted])}2], (IV), also prepared from hydrated iridium(III) chloride and (2-hydroxyphenyl)di-t-butylphosphine in isopropyl alcohol. Complex (IV) in benzene slowly reacts with air to give the IrII complex or rapidly with carbon monoxide to give colourless [[graphic omitted])}2]. A benzene solution of trans-[[graphic omitted])}2] in air slowly (24 h) gives the purple five-co-ordinate C-metallated IrIII complex [[graphic omitted])}]; this readily takes up carbon monoxide in the sixth co-ordination position to give a colourless adduct. Various other reactions to give unidentified products are described. 1H and 31P n.m.r. and i.r. data are given.